Field of the Invention
The invention relates to a method of adjusting dampening-solution feed in an offset printing press by a control device.
Offset printing presses of the type mentioned herein comprise an inking unit and a dampening unit, which are preset before each new print job in order to minimize the start-up wastage, i.e., in order to obtain the first so-called good sheet as quickly as possible. In conventional inking units, zone presetting is often performed, wherein, during a changeover of the press, the zones are opened wide at locations whereat, depending upon the subject, more ink is picked up. In the areas wherein many nonprinting locations appear on the printing form and, therefore, only little ink is required, the zones are only slightly opened.
With regard to short inking units, such as anilox inking units, this zone presetting is omitted, because these inking units operate without zones and are completely free of ghosting. The ink density is accordingly correct from the start of the print job, irrespective of which printing form is put in place. In the case of such a short inking unit, only the dampening-solution feed has yet to be adjusted in wet offset. The start-up wastage therefore depends here upon how quickly the correct adjustment for the dampening-solution feed is found. The dampening-solution demand depends upon the subject and the type of paper. More dampening solution is required for printing forms with a high take-up of ink than for light printing forms with little ink take-up. Also, more dampening solution is required for uncoated paper than for coated paper.
In connection with the adjustment of dampening-solution feed in dampening units, characteristic curves, also known as run-up curves, are stored in a control device. The characteristic curves indicate the different dampening-solution demand at different speeds. It is thereby possible to accelerate the printing press after the first good sheet has been printed, the dampening-solution feed being adjusted along the respective characteristic curve.
In a method of adjusting the dampening-solution feed disclosed in the published German Patent Document DE 38 28 182 A1, presetting of the dampening unit is performed manually, i.e., by operating personnel, who change the dampening-solution feed during start-up of the printing operation until the first good sheet is obtained. Following this manual basic setting of the dampening-solution feed, which is performed at a low printing speed, the position of the characteristic curve is defined in a graph wherein the quantity of dampening solution is plotted against machine speed. If the printing speed is then increased, the control device adjusts the quantity of dampening solution required for the respective printing speed, which is to be fed in along the characteristic curve. A disadvantage of the heretofore known method is that adjustment of the dampening unit is performed only after start-up of the print job, the level of start-up wastage being dependent upon the empirical values and the technical knowledge of the operating personnel.
It is an object of the invention to provide a method of adjusting dampening-solution feed in an offset printing machine wherein a production of start-up wastage is reduced in comparison with heretofore known methods of this general type.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a method of adjusting dampening-solution feed of an offset printing press by a control device, which comprises determining, in a first step, a required dampening-solution demand for a first calibration printing form with a known minimum percent area coverage and for a second calibration printing form with a known maximum percent area coverage at a calibration speed, and storing the dampening-solution demand in the control device as interpolation reference points; determining, in a second step, a percent area coverage lying between the maximum and the minimum percent area coverage of the calibration printing forms of a printing form to be used for a forthcoming print job, and passing the determined percent area coverage on to the control device; determining by the control device, in a third step, the dampening-solution demand required for the forthcoming print job with the printing form provided for the purpose at the calibration speed, by interpolating between the interpolation reference points; determining, in a fourth step, a printing-speed dependent characteristic curve of the dampening-solution demand required for the forthcoming print job with the printing form provided for the purpose; and controlling, in a fifth step, during the processing of the print job, the feed of the dampening solution along the characteristic curve.
In accordance with another mode, the method invention further comprises, in the first step, determining the required dampening-solution demand of the first calibration printing form and that of the second calibration printing form at calibration speed empirically.
In accordance with a further mode, the method invention further comprises, in the second step, determining the percent area coverage of the printing form to be used for the forthcoming print job by a plate scanner.
In accordance with an alternative mode, the method invention further comprises, in the second step, determining the percent area coverage of the printing form to be used for the forthcoming print job from image data from a prepress system.
In accordance with an added mode, the method invention further comprises, for a third calibration printing form with a known, mean percent area coverage, determining the respectively required dampening-solution demand at various printing speeds, and determining therefrom a characteristic reference curve.
In accordance with an additional mode, the method invention further comprises storing the characteristic reference curve in the control device.
In accordance with yet another mode of the method invention, the characteristic curve of the dampening-solution demand required for the forthcoming print job with the printing form provided for this purpose, and the characteristic reference curve are identical and run parallel to one another.
In accordance with yet a further mode, the method invention further comprises determining at least one further value of the dampening-solution demand at different print speeds for refining the characteristic reference curve.
In accordance with yet an added mode, the method invention further comprises, during production printing, finely adjusting manually the dampening-solution feed, starting from the characteristic curve, and storing in the control device the curve actually run during the print job and relating to the dampening-solution feed dependent upon the printing speed and the percent area coverage.
In accordance with a concomitant mode, the method invention further comprises, before the start of a subsequent print job, comparing the characteristic curve determined by the control device with the values of the curve previously actually run for the same percent area coverage.
For achieving the object of the invention, the method is distinguished by the fact that, first, the required dampening-solution demand is determined for a first calibration printing form with a known minimum percent area coverage and for a second calibration printing form with a known maximum percent area coverage at a calibration speed and is deposited in the control device as interpolation reference points. Then, the percent area coverage lying between the maximum and the minimum percent area coverage of the calibration printing forms, of a printing form to be used for a forthcoming print job is determined and passed on to the control device. In a third step, with the aid of the control device, the dampening-solution demand required at the calibration speed for the forthcoming print job with the printing form provided for the purpose is determined by interpolation between the interpolation reference points. In a fourth step, a printing-speed dependent characteristic curve of the dampening-solution demand required for the forthcoming print job with the printing form provided for the purpose is determined. Finally, in a fifth step, during the processing of the print job, the dampening-solution feed is controlled along the characteristic curve. Likewise, different curves are stored for different types of paper. Machine-finished papers require a great deal of dampening solution, coated papers need less dampening solution. The method offers the advantage that, with the aid of the control device, presetting of the dampening unit can be performed as a function of a subject and of the size of the ink-carrying area on the printing form, respectively. The dampening unit can therefore be adjusted even before the start of the printing operation so that, in particular, in short inking units, preferably no start-up wastage, but at least only relatively low start-up wastage occurs, in comparison with heretofore known methods of this general type. The first so-called good sheet is therefore obtained relatively quickly. The control device therefore calculates the starting adjustment or adjustments of the dampening unit and, during the processing of the print job, matches the dampening-solution feed to the respective printing speed. In the method according to the invention, therefore, the extent of start-up wastage does not depend upon the knowledge of the operating personnel, who at most intervene in the printing process for the purpose of making a fine adjustment.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method of adjusting damping-solution feed in an offset printing press, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.